1. Field of the Invention
The present invention relates to a control method for enabling a welding torch to trace a weld line while oscillating the welding torch by detecting an electric change responsive to changes in an arc length and a wire extension.
2. Description of Related Art
Conventionally, there has been widely used a control method for tracing a welding line according to a detection signal of an arc sensor for detecting an electric change responsive to changes in the arc length and the wire extension caused when the welding torch is oscillated.
The conventional arc sensor can yield relatively good results in the horizontal fillet welding provided that the welding speed is relatively low, for instance, of an order of 120 cms/min.. Also, in the horizontal fillet welding, the arc sensor is applicable to a thin plate of an order of 2 mms in thickness. In the conventional tracing method as mentioned above, the oscillation frequency of the welding torch is set at a relatively low frequency in the range from 0.5 to 5 Hz.
Referring to FIGS. 1 and 2, a relationship between each position of the oscillation of the welding torch and the welding current detected when the welding torch is oscillated at a relatively low frequency between left and right works Wl and Wr to be welded will be described below.
The portion (B) of FIG. 1 is a schematic front view of the welding torch WT and the left and right works Wl and Wr when the center C of the oscillation coincides with the weld line W.sub.0. In the portion (B) of FIG. 1, A.sub.0 and A.sub.1 denote lengths (referred to as wire extensions hereinafter) of consumable electrodes projecting from the tip end WTa of the welding torch WT, and B.sub.0 denotes a distance (referred to as an arc length hereinafter) defined between the end Wa of the consumable electrode (referred to as a wire hereinafter) W and respective surfaces of the works. The end Wa of the wire W is referred to as a wire end Wa hereinafter.
Referring to the portion (B) of FIG. 1, when the welding torch WT is located at a left edge L.sub.0 of the amplitude of the oscillation, the wire extension is A.sub.1 and the arc length is B.sub.0. As the welding torch WT moves from the left edge L.sub.0 to the center C of the oscillation corresponding to the weld line W.sub.0 at a low oscillation frequency, the welding current increases according to the self-control action of a welding power source based on a regulated constant voltage characteristic, the melting speed of the wire increases, and thereby, the arc length becomes approximately constant B.sub.0. Therefore, the wire extension increases gradually from the length A.sub.l to the length A.sub.0.
As shown in the portion (A) of FIG. 1, when the wire extension increases, the average value I of the welding current which is represented by the axis of ordinate decreases from a current I.sub.1 to a current I.sub.0 according to a change in the position P of the oscillating welding torch WT which is represented by the axis of abscissa. On the other hand, when the welding torch WT moves from the center C of the oscillation to the right edge R.sub.0 thereof, the wire extension decreases from the length A.sub.0 to the length A.sub.1 while keeping the arc length B.sub.0 constant contrary to the aforementioned action, and the welding current increases from the current I.sub.0 to the current I.sub.1. Therefore, when the center C of the oscillation coincides with the weld line W.sub.0, namely, when there is no shift between the center C of the oscillation and the weld line W.sub.0, respective characteristics of the welding current detected on both sides of the weld line W.sub.0 are symmetric, namely, the welding current detected when the welding torch WT moves in the left direction is the same as that detected when the welding torch WT moves in the right direction.
The portion (C) of FIG. 1 shows a relationship between the welding current I represented by the axis of ordinate and a passed time t represented by the axis of abscissa, and the portion (D) of FIG. 1 shows a relationship between the position P of the oscillating welding torch WT represented by the axis of ordinate and a passed time t represented by the axis of abscissa.
The portion (B) of FIG. 2 is a schematic front view of the welding torch WT and the left and right works Wl and Wr when the center C of the oscillation is shifted from the weld line W.sub.0. As shown in the portion (B) of FIG. 2, as the welding torch WT is oscillated from the left edge L.sub.1 of the amplitude of the oscillation through the center C thereof to the right edge R.sub.1 thereof, after the wire extension increases from a length A.sub.3 to a length A.sub.0 corresponding to the weld line W.sub.0 while keeping the arc length B.sub.0 constant as well as the action shown in FIG. 1, the wire extension decreases from a length A.sub.0 through a length A.sub.4 corresponding to the center C of the oscillation to a length A.sub.5. Therefore, when the center C of the oscillation is shifted from the weld line W.sub.0, responsive characteristics of the welding current I detected on both sides of the weld line W.sub.0 are asymmetric, namely, respective welding current detected when the welding torch WT moves in the left direction is the same as that detected when the welding torch WT moves in the right direction.
The portion (C) of FIG. 2 shows the relationship between the welding current I represented by the axis of ordinate and a passed time t represented by the axis of abscissa, and the portion (D) of FIG. 2 shows the relationship between the position P of the oscillating welding torch WT represented by the axis of ordinate and a passed time t represented by the axis of abscissa.
Next, the relationship between the average value Ia of the welding current and the distance D in the direction toward the wire end Wa between the tip end WTa of the welding torch WT and the surface of th works Wl and Wr will be described below with a well known experimental equation.
Generally, the melting speed Vm of the wire in the arc welding is represented by the following equation: EQU Vm=a.sub.0 .multidot.Ia+b.sub.0 .multidot.A.multidot.(Ia).sup.2( 1)
wherein
Ia is the average value of the welding current I, PA1 A is the wire extension, and PA1 a.sub.0, b.sub.0 and c.sub.0 are constants, respectively. PA1 detecting said electric signal as a third detecting signal when said welding torch moves in the left direction for a second time interval of the left half of the amplitude of the oscillation;
Since the arc length B.sub.0 is approximately constant at a low oscillation frequency, the melting speed Vm of the wire is assumed to be equal to a feeding speed Vf of the wire, resulting in the following equation. EQU Vm=Vf=g (2)
wherein g is a constant.
The following equation is obtained by combining the equations (1) and (2). EQU a.sub.0 .multidot.Ia+b.sub.0 .multidot.A.multidot.(Ia).sup.2 =Vm=Vf=g(3)
The following equation is obtained by differentiating the equation (3) by a time t. EQU a.sub.0 .multidot.dIa+b.sub.0 {(Ia).sup.2 .multidot.dA+2A.multidot.Ia.multidot.dIa}=0 (4)
On the other hand, the aforementioned distance D is represented by the following equation: EQU D=B.sub.0 =A (5)
Since the arc length B.sub.0 is smaller than the wire extension A and is approximately constant, D.perspectiveto.A. Therefore, the following equation is obtained by substituting the distance D with the wire extension A in the equation (3). ##EQU1##
Since there is no term depending on a time in the equation (6), the welding current Ia is determined by setting the distance D determined by the position P of the oscillating welding torch WT. Namely, the distance D measured when the welding torch WT moves in the right direction from the left edge of the amplitude of the oscillation to the right edge thereof is the same as the distance D measured when the welding torch WT moves in the left direction from the right edge of the amplitude of the oscillation to the left edge thereof, the characteristic of the welding current detected when the welding torch WT moves in the left direction is substantially same as that detected when the welding torch WT moves in the right direction, and the characteristic of the welding current detected when the welding torch WT is located in the left half of the amplitude of the oscillation is substantially same as that detected when the welding torch WT is located in the right half thereof. This implies that the action described above based on the equation (6) is similar to that described above referring to FIGS. 1 and 2.
In the conventional control process, the weld line W.sub.0 is determined based on the action described above referring to FIGS. 1 and 2 or the equation (6) using either one of the following methods from (a) to (d).
(a) An integrated value of the welding current detected for the time interval of the right half of the amplitude of the oscillation is compared with an integrated value of the welding current detected for the time interval of the left half thereof so as to detect the weld line W.sub.0.
(b) A peak value of the welding current detected when the welding torch WT is located at a specific position of the right half of the amplitude of the oscillation is compared with a peak value of the welding current detected when the welding torch WT is located at a specific position of the left half thereof so as to detect the weld line W.sub.0.
(c) A position of the welding torch WT when a minimum welding current is detected during oscillating the welding torch WT is detected as the weld line W.sub.0.
(d) The weld line W.sub.0 is detected by using a method of a modification or a combination of the aforementioned methods (a) to (c).
Since a change in the melting speed of the wire follows a change in the position of the oscillating welding torch WT at a low oscillation frequency of 0.2 to 5 Hz in the conventional control process so as to keep the arc length B.sub.0 approximately constant, the welding current follows a change in the wire extension A or a change in the distance D and varies under the condition of the characteristics shown in FIGS. 1 and 2 or the characteristic indicated by the equation (6). Therefore, the position P of the oscillating welding torch WT can be controlled satisfactorily with use of one of the aforementioned control methods (a) to (c) so as to trace the weld line W.sub.0.
The aforementioned control method can be used even in a horizontal fillet welding for welding thin plates having a thickness of about 2.0 mms. However, in order to weld thin plates having a thickness of about 1.6 mms, it becomes necessary to increase the welding speed, namely, to heighten the oscillation frequency. For example, when the oscillation frequency is set so as to fall within the range from 0.2 to 5 Hz in such a case, the welding bead may be form thereon with meandering because of the low oscillation of the welding torch WT. In order to prevent the welding bead from being formed thereon with meandering, it is necessary to set an oscillation frequency equal to or higher than 5 Hz. When the oscillation frequency becomes about 5 Hz, the melting of the wire at the melting speed Vm corresponding to a change in the distance A+B defined between the tip end WTa of the welding torch WT and the surface of the works Wl and Wr, can not follow a change in the position P of the oscillating welding torch WT since the position P thereof varies rapidly. Therefore, the arc length B varies, resulting in that the real change in the welding current can not reflect the characteristics shown in FIGS. 1 and 2 or the characteristic represented by the equation (6).
Namely, in the case of no shift between the center C of the oscillation and the weld line W.sub.0, when the welding torch WT moves from the left edge L.sub.0 of the oscillation to the right edge R.sub.0 thereof, the wire end Wa draws a locus of a curve as shown in the portion (B) of FIG. 3. The reason why the wire end Wa draws this locus is as follows. At an oscillation frequency of about 5 Hz, when the aforementioned distance A+B increases gradually, the decrease in the melting speed Vm of the wire can not follow a change in the distance A+B due to a change in the position P of the oscillating welding torch WT, resulting in that the arc length B can not be kept constant. Therefore, as shown in the portion (B) of FIG. 3, after the arc length increases from a length B.sub.0 to a length B.sub.2, the welding torch WT passes through the center C of the oscillation, and then, the arc length decreases down to a length B.sub.3. Thereafter, the arc length increases up to a length B.sub.0. Responsive to the above change in the arc length, the welding current decreases from a current I.sub.5, and when the welding torch WT passes through the center C of the oscillation, the welding current becomes a current I.sub.6. Furthermore, the welding current decreases down to a current I.sub.7, and thereafter, the welding current increases up to a current I.sub.5 detected when the welding torch WT is located at the right edge of the oscillation.
Thereafter, when the center C of the oscillation is shifted from the weld line W.sub.0, the wire end Wa draws a locus of a curve as shown in the portion (B) of FIG. 4 when the welding torch WT is oscillated. According to the above change in the position of the wire end Wa, the welding current draws a locus of a curve as shown in the portion (A) of FIG. 4.
Thus, when the oscillation frequency reaches about 5 Hz, a substantial change reflecting to the shift amount between the center C of the oscillation and the weld line W.sub.0 can not be obtained in the conventional method for comparing an integrated value, a peak value, or a minimum value for one period of the welding current detected for the time interval of the right half of the amplitude of the oscillation, with that detected for the time interval of the left half thereof, resulting in that it has been difficult to put the welding apparatus utilizing the above control method in practical use.
In order to solve the above problems, there have been proposed two following methods. One is a method for detecting a weld line W.sub.0 by comparing a difference between a minimum value of a welding current detected when the welding torch WT moves in the right direction and a welding current detected when the welding torch WT is located at the right edge of the oscillation, with a difference between a minimum value of a welding current detected when the welding torch WT moves in the left direction and a welding current detected when the welding torch WT is located at the left edge of the oscillation. Another is a method for detecting a weld line W.sub.0 by comparing an integrated value of the welding current detected for the time intervals of the left and right halves when the welding torch WT moves in the right direction, with an integrated value of welding current detected for the time intervals of the left and right halves when the welding torch WT moves in the left direction.
These improved methods can be applied to the case at a low oscillation frequency of about 5 Hz. However, at an oscillation frequency higher than about 5 Hz, as described later, the locus of the welding current to the position P of the oscillating welding torch WT further changes therefrom. Therefore, the difference between the detected values becomes small, resulting in that it is difficult to detect the weld line W.sub.0 precisely.
When the characteristics of the action shown in FIGS. 1 or 2 or represented by the equation (6) are applied to the conventional control method for tracing the weld line W.sub.0 at a low oscillation frequency of 0.5 to 5 Hz, it is assumed that the arc length B.sub.0 is kept constant independent of the position P of the oscillating welding torch WT. Therefore, if the characteristic of the position P of the oscillating welding torch WT or the distance D in the direction toward the wire end Wa defined between the tip end WTa of the welding torch WT and the surface of the works Wl and Wr when the welding torch WT moves in the right direction is substantially same as that when the welding torch WT moves in the left direction, the aforementioned conventional control method can be used only under the condition that the characteristic of the welding current detected when the welding torch WT moves in the right direction is substantially same as that when the welding torch WT moves in the left direction.
Therefore, in the conventional control method, the welding speed is limited to about 120 cms/min. at the most in the case of a horizontal fillet welding, and the welding speed is limited to about 80 cms/min. at the most in the case of a lap fillet welding. The minimum thickness of plates which can be welded is limited to 2.0 mms in the case of the horizontal fillet welding, and it is limited to 3.2 mms in the case of the lap fillet welding.
Furthermore, in the case of a vertical down welding utilizing the conventional control method, since a speed at which the melting metal drops is higher than the welding speed, it is difficult to put the vertical down welding in practical use. The same control method can not be applied to both a spray arc welding and a short arc welding, namely, separate control methods must be applied to them, respectively. Therefore, a use of each of the conventional control methods is limited. Due to this, the conventional control methods can not meet the latest needs of the automation in a welding at a high oscillation frequency and in a vertical welding for welding thin plates for forming respective portions of an automobile.